For patients with acute myeloid leukemia, allogeneic hematopoietic stem cell transplantation is an important and sometimes life-saving therapy. However, the relapse rates are very high at 40 to 70 percent. An infection with cytomegalovirus (CMV) poses a further threat to transplant recipients. But if the virus reactivates after transplantation, it may also protect against a leukemia relapse. To better understand this controversial observation, scientists led by Xiao-Hua Luo and Per Ljungman from the Karolinska Institutet in Stockholm, Sweden, examined the peripheral blood mononuclear cells of leukemia patients after stem cell transplantation. Using FACS analysis and peptide pools from peptides&elephants, they tested whether reactivation of the cytomegalovirus influences T cell immunity against Wilms tumor 1 - and thus against leukemia cells. Peptide pools represent a complete protein by mapping its primary amino acid sequence into individual 15mer peptides, each overlapping by 11 amino acids. The scientists published their results in Frontiers in Immunology.
Cytomegalovirus: threat or protection after transplantation?
Since medical professionials began using antiviral treatments to prevent cytomegalovirus infection after stem cell transplantation, fewer patients fell ill with the virus and the mortality rate decreased significantly. However, reactivation of cytomegalovirus may also reduce the risk of relapse in patients with acute myeloid leukemia, as a growing body of research results shows.
According to previous research, the reactivation of the cytomegalovirus may increase T cell immunity against leukemia cells. To test this hypothesis, Luo and coworkers compared cytomegalovirus-specific and Wilms tumor 1-specific T cells in the peripheral blood of patients with acute myeloid leukemia.
The scientists included 24 adult patients with acute myeloid leukemia in the first or second complete remission in their study. All patients had received allogeneic hematopoietic cell transplantation and were Wilms tumor 1 positive before cell transplantation. The scientists collected peripheral blood mononuclear cells from the patients at 1, 2, 3, 6 and 12 months after transplantation.
Wilms tumor 1-specific T cells fight leukemia cells
Most cells of acute myeloid leukemia overexpress the Wilms tumor 1 gene, which plays an important role in cell proliferation, differentiation and apoptosis. Therefore, the expression of Wilms tumor 1 mRNA in peripheral blood and bone marrow is considered a marker for residual disease in acute myeloid leukemia.
The more Wilms tumor 1-specific T cells a patient has, the stronger his or her immune system fights the leukemia. Allogeneic hematopoietic stem cell transplantation and vaccination with Wilms tumor 1 peptide can stimulate the cytotoxic activity of Wilms tumor 1-specific CD8+ T cells.
CMV-specific T cells recover faster and respond to Wilms tumor 1 protein
Luo and coworkers showed in their study: Cytomegalovirus-specific CD8+ T cells reconstituted much faster than the Wilms tumor 1-specific CD8+ T cells in the measured time period. Furthermore, the cytomegalovirus-specific T cells expressed fewer exhaustion markers and were more functional, as shown by the production of IFN-γ/TNF-α and the expression of Eomes/T-bet.
The scientists stimulated the patients' T cells with the cytomegalovirus pp65 protein or the Wilms tumor 1 protein – both peptide pools from peptides&elephants. They then used FACS analysis to determine what phenotype they had and whether the CD8+ or CD4+ T cells produced IFN-γ and TNF-α. The results: If the cytomegalovirus had reactivated within the measured time period, more cytomegalovirus-specific CD8+ T cells were produced – in response to both proteins. At the same time, the expression values of Eomes+T-bet- were lower and those of Eomes+T-bet+ were higher. Cytomegalovirus-specific T cell clones are therefore able to respond to Wilms tumor-1 as a leukemia antigen stimulus.
15-mers of Wilms tumor-1 identified that trigger T cell response
In addition, the scientists mapped immunogenic epitopes of Wilms tumor 1 that are recognized by cytomegalovirus-specific T cells. To do this, the scientists stimulated aliquots of cytomegalovirus-specific T cells with individual Wilms tumor 1 peptides. They used survivin peptide pool from peptides&elephants as a control. After 72 hours of stimulation, they collected the supernatants and assessed the concentration of IFN-γ using ELISA. With this mapping grid they were able to identify specific 15-mers of Wilms tumor 1 that trigger a T cell response.
CMV-specific T cells react to leukemia antigen – cross-reaction with leukemia cells remains unclear
The scientists' data showed: The cytomegalovirus-specific T cells recovered more quickly after a transplantation and produced more cytokines in response to stimuli than the Wilms tumor 1-specific T cells. If the cytomegalovirus had reactivated within the measured time period, more cytomegalovirus-specific CD8+ T cells were produced when stimulated with a cytomegalovirus protein or Wilms tumor 1. Cytomegalovirus-specific T cell clones therefore react to a leukemia antigen. However, whether the cytomegalovirus-specific T cells are able to cross-react with leukemia cells still needs to be investigated.
Luo XH et al.
Different recovery patterns of CMV-specific and WT1-specific T cells in patients with acute myeloid leukemia undergoing allogeneic hematopoietic cell transplantation: Impact of CMV infection and leukemia relapse.